The present invention relates to an improvement in a plate fin heat exchanger assembled integrally by brazing, wherein an aluminum alloy suppressing an excessive diffusion of silicon of brazing alloy into fins is used for a fin serving as a superhigh-pressure fluid passage.
Conventionally, AA3003 alloy with a good brazability has been extensively used as fin stock in plate fin heat exchangers fabricated by brazing using aluminum alloy. However, when fins are designed for use under a superhigh pressure of 55 kg/cm.sup.2 G or more, AA3003 alloy is inadequate as fin stock because of an insufficient tensile strength. Thus, under the such superhigh pressure, AA3004 alloy has been used instead of AA3003 alloy. AA3004 alloy has approximately one-half times higher strength than AA3003 alloy and a sufficient formability as fin stock.
Fins made of AA3004 alloy are normally brazed at a temperature of 580.degree. to 610.degree. C., using an aluminum-silicon brazing alloy containing, by weight, about 6.8 to 13% silicon. In fabricating heat exchangers of high density wherein fins having an increased plate thickness and corrugated at fine corrugation pitch are incorporated, it takes a very long time to preheat parts to be brazed and then heat uniformly all of them to the above brazing temperature. Thus, in brazed parts of the fin which reach the brazing temperature for a relatively short heating time, and are in contact with molten brazing alloy and then in the liquid-phase state for a long time, an unfavorable excessive diffusion of silicon contained in the brazing alloy into the fins is apt to occur. As a result of the excessive diffusion, the width of the brazed joint will be progressively reduced and bonding strength will be seriously decreased.
With respect to AA3004 alloy used as fin stock, it has been said that Mg contained in the fin in an amount of approximately 1 wt.% has a tendency to promote the diffusion of silicon in the brazing alloy into the fin.